The present invention relates to a carrier for use in a developing device of a latent electrostatic image and production thereof.
In a known developing process in which a magnetic brush is formed on a surface of a developing sleeve by magnetic intensity, a surface of a photosensitive member functioning as a supporter of the latent image or the like is rubbed by a magnetic brush of the magnetic developer, and then the latent image thereon is developed. In the above developing process a mixture of magnetic carriers consisting of iron particles having an average particle size of about 20-200 micrometers and insulating toners having an average diameter of about 5-20 micrometers had been used as the developer.
Such carriers, however, tend toward many problems such as disorder of the latent image and defect of the developed image due to the escape of an electrical charge on the supporter of the latent image through the carrier or adhesion of the carriers into the image parts of the supporter by the injected electrical charge from the developing sleeve when the toner content in the developing agent decreases through continuous use, because the volume specific resistivity of the carrier itself is generally too low, such as less than 10.sup.8 ohm.cm. Furthermore, when the carriers adhere to the latent image on the photosensitive member, the surface of the member is liable to be damaged due to the hardness of the carriers, at the cleaning of the surface by a blade cleaner and the like.
In order to solve the above problems resin coated carriers had been proposed to give a higher resistivity to such carriers in, for instance, Japanese Patent Publication (Kokai) No. 66264/1985, Japanese Patent Publication (Kokai) No. 66265/1985, Japanese Patent (Kokai) No. 660/1982, Japanese Patent Publication (Kokai) No. 60658/85 and so on. The carriers disclosed in the above prior arts are generally produced by coating carriers with a resin solved in a suitable solvent, and drying them.
These carriers have a resin coating layer thereon, but have still many problems such as difficulty of quality control of image attributed to accumulation of electrical charge on the carriers, residual solvent in the carrier core, and low electrical resistance. The electrical resistance can be increased by thickening the resinous coating layer, but it increases the cost of the carrier, because plural coatings will be required to obtain such a thicker layer.
Further, according to the above methods numerous fine pores are formed in the coated resin layer when the solvent vaporizes, and then the layer is liable to peel off around the pores, so that the durability is lower.
Furthermore, the carrier coated with the resinous solution is so unstable with respect to chargeability that the high density of the copied image cannot be achieved repeated use because the charge amount is increased by it.
The toner transported from the developing sleeve to the part of electrostatic latent image is released from the carrier surface to the latent image to make it visible. At that time as copy of an even and broad black area cannot be achieved by the self-bias of the latent image alone because of the edge effect, the quality of the image must be controlled by means of a bias voltage applied from outside. Therefore, in order to apply the bias evenly and effectively a partial electroconductivity must be given on the surface of the carriers. However, carriers having a partial electroconductivity are difficult to obtain, and the high electrical resistance and the partial conductivity are incompatible by the conventional methods.
In the case that non-treated iron powders or ferrites are used as carriers the application of bias is possible, but such carriers have defects such as a lower electrical resistance and shorter life.